Ultra-high purity (UHP) inert gases such as argon and nitrogen are important in certain industries. For example, in the electronics industry, UHP nitrogen and argon are critical in the successful manufacture of silicon wafers. The following table shows the typical maximum allowable concentrations of contaminants in UHP nitrogen:
______________________________________ Impurity Maximum Allowable Concentration ______________________________________ Oxygen 5 ppb Carbon Dioxide 10 ppb Carbon Monoxide 5 ppb Water 100 ppb Methane 10 ppb ______________________________________
A major challenge is to develop materials and processes which will produce UHP gases in an efficient and cost-effective manner. Removal of impurities to ppb levels, as well as accurate sampling and analysis to determine the levels, is very difficult.
There are many techniques currently available to remove trace oxygen from inert gases but they have limitations. For example, metal alloys consisting of various metals including zirconium, aluminum, vanadium and iron have been used as high temperature oxygen getters. To activate the alloy, it is heated at 250.degree. C.-900.degree. C. under vacuum. After activation, the preferred temperature of operation is between 200.degree. C. and 400.degree. C. The getter can be used at ambient temperature, however, the oxygen gettering capacity is significantly reduced. These gettering materials are very expensive and their oxygen capacity cannot be regenerated. U.S. Pat. No. 5,194,233, for example, discloses this type of oxygen getter.
Reduced transition metal oxide catalysts, such as copper oxide and nickel oxide in the reduced form, are another group of materials which are used to remove oxygen from gas streams. Production of the reduced form of the catalysts is a strongly exothermic reaction and requires heating in a reducing atmosphere, usually hydrogen, at 150.degree. C.-200.degree. C. Once the oxygen capacity of the catalyst is exhausted, the catalyst must again be reduced in a reducing gas at elevated temperature. Major drawbacks for this type of catalyst are the safety hazards associated with handling the highly pyrophoric catalyst when it is in the reduced state and the requirement of hydrogen for catalyst reduction.
Deoxo catalysts are used to remove oxygen from gas streams via catalytic reaction of oxygen with hydrogen to form water. These systems require hydrogen in amounts exceeding stoichiometric amounts; therefore, water removal and hydrogen carryover must be addressed.
Use of adsorbents to remove impurities from gaseous streams are well known. For example, U.S. Pat. No. 4,271,133 discloses use of a zinc oxide adsorbent to remove hydrogen cyanide from a gaseous stream at a temperature of about ambient to about 350.degree. C. The adsorbent comprises zinc oxide and no more than 5%, by weight, of an oxide of an alkali or alkaline earth metal.
U.S. Pat. No. 4,433,981 discloses use of an adsorbent for carbon dioxide removal from gaseous streams. The stream is contacted with an adsorbent prepared by impregnating a porous aluminum oxide support with an alkali metal or alkaline earth metal oxide or salt which is decomposable upon calcination and subsequently calcining the impregnated alumina at about 350.degree. C.-700.degree. C. to convert the impregnating compound to the corresponding alkali or alkaline earth metal aluminate. After using it to remove carbon dioxide, the adsorbent can be regenerated by heating to calcining conditions.
U.S. Pat. No. 4,579,723 discloses a two-bed system for removing parts per million levels of impurities such as oxygen, carbon monoxide, carbon dioxide, hydrogen and water, from an inert gas stream. The beds are comprised of reactive/adsorbent material; for example, a catalytic material such as DeOxo A (a mixture of chromium and platinum on gamma-alumina) is in the first bed and a getter material such as Dow Q1 (a mixture of copper, nickel and cobalt with traces of silver, chromium and manganese mounted on granular alumina) is in the second bed.
U.S. Pat. No. 4,594,231 discloses removal of halogens and/or hydrogen halides from gases by contacting the gas with an adsorbent comprising an activated carbon support on which are deposited two or more components from the following three groups and one or more components from the remaining two groups: (1) copper compounds; (2) zinc compounds; and (3) alkali or alkaline earth compounds, or compounds of aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, chromium or lead. If compounds other than oxides are deposited on the activated carbon, heat treatment such as drying or calcining after deposition is preferred.
U.S. Pat. No. 4,859,438 discloses a method of separating impurities, such as low levels of sulfur dioxide, hydrogen chloride, and nitrogen oxides, from flue gases by contacting the gases with at least one substantially dry particulate adsorbent including NaHCO.sub.3, which at a release temperature below 400.degree. C., decomposes to form an activated adsorbent including Na.sub.2 CO.sub.3.
U.S. Pat. No. 5,015,411 discloses a scavenger for removing Lewis acid and oxidant impurities from inert gases comprising an inert inorganic support and an active scavenging species on the support. The scavenger is formed by deposition of an organometallic precursor on the support and subsequent pyrolysis of the organometallic material to yield metal hydrides and/or active metals as the active scavenging species on the support.
U.S. Pat. No. 5,081,097 discloses a copper modified carbon molecular sieve for selective removal of all concentrations of oxygen in gases at temperatures up to about 200.degree. C. and trace amounts of oxygen in gases at temperature up to about 600.degree. C. The carbon molecular sieves are regenerated by reduction with hydrogen.